The present invention relates to a low pressure gas discharge switch, in which, for a low-pressure gas discharge, main electrodes are arranged at least at a distance d from each other. The electrodes in an arcing chamber form a cathode and an anode of a discharge path for the low-pressure gas discharge that is triggered by increasing the electron density in a cathode cavity. At least the cathode in its disk-shaped area has at least one aperture for triggering the discharge. The cathode and anode apertures being opposite, and aligned with, each other. An arrangement for generating a magnetic field is assigned to the main electrodes.
Low-pressure gas discharge switches for switching of high pulse-shaped currents and power outputs are essentially composed of at least two main electrodes, of which at least the cathode has one or a plurality of apertures which are designated as trigger apertures. Via this (these) aperture(s), the area between the main electrodes is connected to the area behind the cathode. In this cathode rear space, a trigger device is generally arranged, with whose assistance electrons are released which initiate, i.e., trigger, the necessary main discharge in the area between the anode and cathode, to close the switch.
In switches having thermionic electron generation, i.e., a thyratron, there is, in the cathode rear space, an electrically heated electrode which not only makes the necessary electrons available for triggering the main discharge, but also supplies the greater part of the overall current during the main discharge and thus acts as a thermionic cathode. After each use of the thyratron, however, a significant part of the current continues to flow via the cold cathode and, as a result of vaporization and atomization of the electrode material, leads to erosion of the material.
In low-pressure gas discharge switches, such as those described in WO 89/00354 A1 or German Patent No. 28 04 393 A1, the entire current of the high-current main discharge flows via the cold cathode and leads there to increased erosion, which, in widening the trigger apertures, leads to the destruction of the cathode and thus to the end of the service life of the switching tube. The erosion, within certain limits, is proportional to the entire charge quantity transported by the switch, the quantity thus decisively influencing the service life of the switch. In response to high current densities, i.e., given a small cross-section of the discharge, the erosion rate increases disproportionately; in addition, if the discharge has a small discharge cross-section, a high local volume erosion rate has significantly greater effects than if the discharge has a large discharge cross-section.
The main problem for achieving a long service life of such switching systems is therefore to make the discharge cross-section as large as possible and to provide for a homogeneous current distribution over the entire discharge cross-section. In this way, the erosion is reduced locally and, overall, is distributed equally over a larger surface, so that the result is a uniform wearing away of the electrode instead of locally pronounced erosion. Furthermore, by increasing the discharge cross-section it is achieved that the greater part of the vaporized or atomized electrode material is deposited again on the electrode opposite, so that by increasing the discharge cross-section, a disproportionate reduction of the macroscopically detectable erosion can be achieved.
Low-pressure gas discharge switches are known in various specific embodiments. Specific embodiments having only one, particularly round, aperture in the cathode are also called pseudo-spark switches, and are described in, e.g., WO 89/00354 A1 and German Patent No. 28 04 393 A1. Especially in the switches shown there, there is an aperture in the anode that is identical to, and aligned with, the aperture in the cathode, for maintaining a symmetrical arrangement independent of polarity.
The parallel connection of such individual discharge channels for reducing the load of the individual switching channel is conventional, and it is specifically described in the specialized literature for accommodating the individual channels in a common vacuum housing. The accommodation of individual discharge channels in separate housings is also conventional. Slot-shaped apertures for enlarging the electrode surface are described in German Patent No. 42 40 198 C1. Finally, it is conventional to use a plurality of slot-shaped apertures in the cathode of thyratrons which are operated predominantly using a cold cathode in the so-called xe2x80x9cgrounded-gridxe2x80x9d mode.
A further method for increasing the discharge cross-section is described in detail in U.S. Pat. No. 5,146,141. There, the enlargement of the discharge cross-section is achieved because in the cathode, instead of an aperture, a recess is provided, over whose surface the discharge spreads out, given suitable geometry. The triggering of the discharge is achieved by holes in the edge region of the recess, which connect the actual discharge chamber, between the anode and the cathode, to the cathode rear space and a trigger device accommodated there.
Common to all hitherto known embodiments is that wear and tear takes place through the erosion of the cathode, the erosion, at a time point that cannot be predicted in advance, becoming spatially inhomogeneous, i.e., becoming locally intensified.
This is particularly caused by the self-generated magnetic field of the discharge, as a result of which the discharge tends to be constricted (so-called pinch effect). Since the discharge is triggered due to the low working pressure, preferably assuming the largest aperture, therefore, given the existing asymmetry, a certain point, already in the triggering of the discharge, is favored at which the discharge then burns in a concentrated fashion and at which therefore the erosion becomes more intense. In this way, this undesirable effect intensifies and the service life of the switching element is prematurely limited by local erosion.
A gas discharge switch is also described in Japanese Patent No. 5,159,851. In this gas discharge switch, a means for generating a magnetic filed in the switch is formed using slot arrangements. The slots in the arrangement run in the same direction and are in the walls of hollow electrodes. This means superimposes a parallel, i.e., axial (with respect to the direction of current in the discharge), magnetic field.
An objective of the present invention is to provide a low-pressure gas discharge switch having a cold cathode such that the erosion, particularly of the cathode, is reduced.
In an first example embodiment of the present invention, main electrodes are provided having disk-shaped bottoms. These main electrodes may be provided with radial slots for avoiding eddy current effects. Additionally, a magnetic field generator is provided which produces a substantially parallel magnetic field, i.e., an axial field, with respect to the direction of current in the discharge. An auxiliary electrode may also be provided for electrically triggering a switching process.
In another example embodiment, the main electrodes are provided with disk-shaped bottoms. These main electrodes are provided with slots that run substantially tangentially or in a spiral shape. The magnetic field generator produces a substantially perpendicular magnetic filed, i.e., a radial field, with respect to the direction of current in the discharge. An auxiliary electrode ma also be provided for electrically triggering a switching process.
The means for generating the magnetic fields are preferably realized through slot arrangements in the cylinders completing the anode, on the one hand, and the cathode, on the other hand. However, it is also possible to arrange the slot arrangements in the power supply conductors to the cathode and/or the anode.
The predominantly axial or radial magnetic fields, with respect to the circular symmetry of the low-pressure gas discharge switch, can be influenced by a corresponding tilt of the slots in the different partial units or the arrangement of the permanent magnets or arrangement of individual coils of various types.
Arcs that are superimposed on magnetic fields and the associated means for generating such magnetic fields are already known in principle from the technology of vacuum switches. Especially in connection with gas discharge switches, such magnetic fields surprisingly generate unexpected advantages, since the damaging erosion of the electrodes is reduced particularly for the continuous operation of a gas discharge switch.
The latter is possible as a result of the knowledge that in the low-pressure range switching plasmas can be expanded, using magnetic fields, two or three orders of magnitude more quickly than with the conventional arrangements of vacuum switches. Only in this way is the use of magnetic fields superimposed on the switching plasma sensible in the case of short pulses, since using the previously known expansion speeds of switching plasmas in a vacuum, i.e., in vacuum arcs, no significant improvement of the switching performance and of the erosion of the electrodes can occur for short, high-current discharges.